Optical gratings are used for light coupling and delivery in a variety of optical systems. For example, in energy assisted magnetic recording (EAMR) electromagnetic radiation (light) is provided from a laser to a grating. Typically, the light provided from the laser is in the optical range of the spectrum. The grating is configured for a particular wavelength in the spectrum. Typically this means that the grating actually functions in a range of wavelengths around the particular wavelength. The grating couples light of the particular wavelength from the laser to a waveguide. The light from the waveguide is typically provided to a near-field transducer (NFT) and used to heat a spot on a magnetic recording media. Data is magnetically written to the spot while the spot is heated.
The coupling efficiency of a grating is a measure of the losses in optical energy between light input to the grating and light output by the grating. A higher coupling efficiency translates to lower losses in the grating. Thus, a higher coupling efficiency is desired. In order to achieve high coupling efficiency in a grating, the geometry of the grating, such as the pitch, depth, and shape of ridges in the grating are closely controlled. Thus, fabrication of a grating includes controls of such features.
FIG. 1 depicts a conventional method 10 for fabricating a conventional grating. The core materials, such as Ta2O5 are deposited, via step 12. A photoresist mask is provided on the core material, via step 14. The photoresist mask has a series of lines interleaved with apertures. The core material is etched, via step 16. Thus, the pattern of the photoresist mask is transferred to the core material.
Although the conventional method 10 may be used, there may be drawbacks. FIG. 2 depicts a conventional grating 50 having a pitch, d. The conventional grating 50 includes core material 54 on a substrate 52. The core material 54 includes a plurality of ridges, such as the ridge 62, interleaved with troughs, such as the troughs 64, 66, and 68. Also shown is a photoresist mask 56 used in fabricating the grating 50. The photoresist mask 56 includes lines 58 interleaved with apertures 60 at the pitch, d. The geometry of the conventional grating 50 may differ from what is desired. More specifically, as can be seen in FIG. 2, the troughs differ. For example, troughs 64, 66, and 68 differ in width, depth, and profile. Further, the depth of the troughs that are achievable may be limited by the pitch of the conventional grating 50. Such differences in the troughs may adversely affect performance of the conventional grating 50. More specifically, the optical efficiency of the conventional grating 50 may degrade.
Accordingly, what is needed is an improved method for fabricating a grating.